processing method
Adds a XmlProcessing node with the provided target and text.
For example, to generate an XML processing element <?xml-stylesheet href="/style.css"?> one would write:
builder.processing('xml-stylesheet', 'href="/style.css"');
Implementation
void processing(String target, Object text) {
_stack.last.children.add(XmlProcessingSyntheticImpl(target, text.toString()));
}